1. Field of the Invention
The present invention relates to the papermaking arts. More specifically, the present invention is a papermaker's fabric for use on the dryer section of the papermachine, such a fabric being commonly referred to as a dryer fabric.
2. Description of the Prior Art
During the papermaking process, a fibrous web is formed by depositing a fibrous slurry on a forming fabric in the forming section of a papermachine. A large amount of water drains from the slurry through the forming fabric during this process, leaving the fibrous web on the surface of the forming fabric.
The newly formed web proceeds from the forming section to a press section, which includes a series of press nips. The fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the fibrous web is subjected to compressive forces which squeeze water therefrom. This water is accepted by the press fabric or fabrics and, ideally, does not return to the web.
The web finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, heated from within by steam. The web is directed in a sinuous path sequentially around each in the series of drums by one or more dryer fabrics, which hold the web closely against the surfaces of the drums. The heated drums reduce the water content of the web to a desirable level through evaporation.
The surface characteristics of the fabrics used in the forming and press sections of the papermachine have a direct bearing on the surface properties of the paper being produced. This is also true in the dryer section, where, as stated above, the dryer fabric holds the web closely against the surfaces of the heated dryer cylinders. To promote drying efficiency by increasing the surface area of the dryer fabric directly in contact with the web, and to reduce the marking of the web by the fabric, the dryer fabrics are typically woven to have surfaces which are as smooth as possible. In recent years, one approach that has been taken to provide dryer fabrics with such surfaces has been to include flat monofilament yarns in their woven structures.
While it is indeed clear that the inclusion of flat monofilament yarns on the paper-contacting surfaces of a dryer fabric increases the contact surface area between fabric and dryer cylinder, and therefore between web and dryer cylinder, such fabrics have been observed to be susceptible to wrinkling both during in-house processing and after installation on the dryer section of a papermachine. This is particularly the case when flat monofilament yarns are next, or contiguous, to one another on the surface of the dryer fabric.
Further, where the flat monofilament surface yarns are used to form seaming loops, the dryer fabrics have unacceptably short lives on the papermachine because of the heat and abrasion to which the surface yarns are exposed in the drying section.
For example, U.S. Pat. No. 4,290,209 shows a dryer fabric woven entirely from monofilament plastic polymeric warp and weft strands, wherein at least the warp strands are flattened in cross-section with the long axis of the flattened section extending parallel to the plane of the fabric. The warp strands extend in the machine direction of the fabric, and are provided in an approximately 100% warp fill, which implies that the warp strands are woven contiguously. The fabric does not include a separate system of machine-direction warp yarns, interior of the surface planes formed by the flat yarns, for use in seaming.
U.S. Pat. No. 4,621,663, and related U.S. Pat. No. 4,749,007, show a fabric for draining and drying paper webs. The fabric is formed by warp threads and weft threads, and further comprises a top layer of interlaced lengthwise strips and weft threads. The lengthwise strips are side-by-side one another. The fabric does not include a separate system of machine-direction warp yarns, protected within the interior of the fabric, for use in seaming.
U.S. Pat. No. 5,103,874, and related U.S. Pat. Nos. 5,117,865; 5,199,467; and 5,238,027, show a papermaker's fabric having a system of flat monofilament machine-direction yarns. The system of machine-direction yarns comprises upper and lower yarns which are vertically stacked. At least the upper machine-direction yarns are flat monofilament yarns woven contiguously with each other to reduce the permeability of the fabric and to lock in the machine-direction alignment of the stacking pairs of machine-direction yarns. A seam for the fabric comprised of loops formed from selected flat machine-direction yarns is provided to render the fabric endless during use in papermaking. The fabric does not include a separate system of machine-direction warp yarns, interior of the surface planes of the fabric, for use in seaming.
The present invention is a dryer fabric which may include flat monofilament yarns, but which is woven in a manner that leaves it less susceptible to the above-noted deficiencies of prior-art fabrics. The flat monofilament yarns are not woven contiguously and are not used to form seaming loops. Instead, a separate system of machine-direction warp yarns, interior of the surface planes of the fabric, is provided for use in seaming.